Selecting and Measuring your Axle
When selecting an axle it is important to determine the axle capacity required for your trailer. Trailer capacity is based on the lowest capacity of the trailer components. This includes but is not limited to: frame, axle, spindles, hubs, bearings, wheels, tires, springs and all hardware. It is important to know what axle type you have when reusing old mounting hardware and for proper tire clearance. This includes straight axles (round or square), drop axles with offset spindles, and torsion axles. Also consider spindle size when reusing existing hubs or drums.
Is the distance from outside of hub mounting flange to outside of hub mounting flange, measured one-half way between top and bottom of hub. The hub face is also know as the mounting surface where the wheel studs are located. Hub face to Hub face can also be called the axle track or the centerline of the tire to the centerline of the tire.
Is the distance from centerline of spindle to bottom of the main axle tube.
NOTE: In straight axles, you measure from the spindle centerline and axle main beam centerline. Therefore there is no drop.
Is the distance between center of one spring to the center of the other.
Spring Center Pads:
Are locating or centering pads that can be welded or left loose and are meant for the leaf springs to mount onto. Springs pads can be located at the top or bottom of the axle main beam. DO NOT drill holes in beam in lieu of using spring pads. This would set up stress concentration points, which will weaken the axle beam and allow water to enter.
Mounting your Axle:
When replacing your trailer axle, determine how it was previously mounted. Under slung mounts have the spring mounted below the axle and over slung mounts have the spring mounted on top (also called reverse axle). Spring seats or centering pads can be mounted over or under the axle. Some axles have "camber" or road set, so there will be a bow in the axle until weight is applied. Cambered axles should be mounted with the center bow on top.